The present invention relates to an apparatus for measuring a range to a subject comprising a first optical system which forms a first stationary image of the subject, a second optical system which forms a second image of the subject, said second image being movable with respect to the first stationary image by rotating a scanning mirror of the second optical system, and a photoelectric converting device which receives said first and second images and produces a signal representing an in-focused condition when said first and second images impinging upon the photoelectric converting device coincide with each other.
Such an apparatus has been known and has been used in conventional range finder camera. Upon taking a photograph with such a camera an operator need not adjust an objective lens or camera lens in dependence upon a distance from the lens to a subject at which the camera lens is aimed and thus photographing operation is very easy and simple.
FIG. 1 shows an embodiment of a known range measuring apparatus for use in an automatically focusing camera. A first image of a subject to be photographed is formed on a first photosensitive element 4a by means of a fixed reflecting mirror 1, a first reflecting surface 2a of a rectangular prism 2 and a first lens 3a, while a second image of the subject is formed on a second photosensitive element 4b via a rotatable scanning mirror 5, a second reflecting surface 2b of the prism 2 and a second lens 3b. The first image is stationary, but the second image is movable by rotating the scanning mirror 5 as shown by an arrow R. Thus when the scanning mirror 5 is rotated, the second image is shifted with respect to the stationary first image. Therefore, when the first and second images impinging upon the first and second photosensitive elements 4a and 4b, respectively, coincide with each other, these elements 4a and 4b produce almost same signals. The rotatable mirror 5 is coupled by means of a suitable link mechanism with a driving mechanism for moving a camera lens along its optical axis. When the outputs from the photosensitive elements 4a and 4b become identical with each other, the rotation of the mirror 5 and thus the movement of the camera lens are stopped. At this time, the camera lens is correctly focused with respect to the subject at which the camera lens is aimed.
In general, such an apparatus is advantageously applied to a so-called compact camera in which a camera lens could not be exchanged, because a position and a size of a range measuring area which is projected on the photosensitive element are always fixed with respect to a field of view, i.e., an image of the scene which can be seen by means of a viewing finder. Usually, a center area of the field of view is selected as the range measuring area. However, it is preferable that the position and/or size of the range measuring area can be changed in case that an attachment lens is to be attached in front of the camera lens, the camera lens is to be focused to a subject which does not situate at the center of the field of view, or the camera lens is to be exchanged by other lenses which have different focal lengths. This will be explained more in detail with reference to FIGS. 2a to 2c. These figures show images of a scene which can be seen via the viewing finder in case of attaching to a camera body a wide angle lens, a standard lens and a telephoto lens, respectively. As illustrated in these figures, an angle of view is successively decreased according to the lenses to be used, but the range measuring area M denoted by a broken line is not changed. That is to say, the position and size of the range measuring range M is fixedly determined with respect to the field of view, i.e., the image which can be seen via the viewing finder. Therefore, when the attachment lens is attached to the camera lens or the camera lens is exchanged by other lenses having different focal lengths, a ratio between the range measuring area M and the field of view is changed to a great extent. This results in that an accuracy of range measurement might be extremely affected, particularly in a proximity photographing. In order to change the size of the range measuring area M it is necessary to change, move or exchange the lenses 3a and 3b arranged in the range finder. However, such a measure results in a complicated and large-scale construction of the range finder.
Moreover, in the known range finder, the position of the range measuring area M is fixed at the center of the field of view and thus the manner of photographing or a composition of photograph is limited, because a subject with respect to which the camera lens is in-focused should be brought always at the center of the image which can be seen through the viewing finder.